Takamichi honma

ABSTRACT

AN IMPROVED PULSE POSITION MODULATOR IS DESCRIBED WHEREIN SELECTED PULSES ORDINARILY REPRESENTATIVE OF THE ZERO SIGNAL LEVEL OF THE INFORMATION TO BE TRANSMITTED AND DELETED. DELETION IS ACCOMPANIED BY GENERATING A PULSE POSITION MODULATED SIGNAL REPRESENTATIVE OF THE INFORMATION SIGNAL AND HAVING A PRESELECTED PULSE REPETITION RATE,   WITH SELECTED PORTIONS OF THE PULSES THEREFROM DELETED BY USE OF ANOTHER PULSE TRAIN OF THE SAME REPETITION RATE BUT DELAYED IN TIME BY AN AMOUNT CORRESPONDING TO THE POSITION OF THE PULSE TO BE DELETED.

United States Patent 27,738 PULSE POSITION MODULATION COMMUNI- CATIONSSYSTEM INCLUDING MEANS FOR SUPPRESSING ZERO-MODULATION SIGNAL COMPONENTSTakamichi Honma, Yasuhiro Toshitsuna, and Sahuro Aoki, Tokyo, Japan,assignors to Nippon Electric Company, Limited, Tokyo, Japan Original No.3,562,671, dated Feb. 9, 1971, Ser. No. 718,146, Apr. 2, 1968.Application for reissue Dec. 22, 1971, Ser. No. 210,717

Int. Cl. H03k 7/04 US. Cl. 332-9 R 10 Claims Matter enclosed in heavybrackets If] appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE An improved pulse position modulator isdescribed wherein selected pulses ordinarily representative of the zerosignal level of the information to be transmitted are deleted. Deletionis accomplished by generating a pulse position modulated signalrepresentative of the information signal and having a preselected pulserepetition rate, with selected portions of the pulses therefrom deletedby use of another pulse train of the same repetition rate but delayed intime by an amount corresponding to the position of the pulse to bedeleted.

The present invention relates generally to a pulse position modulation(PPM) technique applied to wired or wireless communication systems and,more particularly, to an improved pulse position modulation (PPM) systemto be used in the random access discrete address (RADA) communicationssystems.

In the known PPM communication systems, only the pulse positions arevaried in accordance with the input modulating signal voltage, while thepulse amplitudes and widths are kept substantially constant. Also therate of sampling the information signal to be transmitted is made,according to the sampling theorem at least twice as high as thepredicted maximum frequency component contained in the informationsignal. As compared with other types of pulse modulation systems such asthe delta modulation and the pulse code modulation, the pulse positionmodulation has outstanding features such that the average rate oftransmission pulses for securing the equivalent degree of transmissionquality is relatively low or, in other words, that the average pulsespacing in the modulated PPM pulse can be made large. It is thereforepossible to save the mean transmission power and consequently to reducethe interfence with or disturbance to the neighbouring communicationchannels, particularly in the RADA system. The above-mentioned longaverage pulse spacings are also advantageous when applied to the RADAsystems in which each of the transmitted pulses is often virtuallyexpanded in its width at the reception site to the order of severaltimes as large as the Original pulse on account of the so-calledmultipath transmission effect. To take advantage of thesecharacteristics PPM transmission has found widest application in theRADA system, in which a few frequency channels are shared by a pluralityof communication stations.

According to a statistical analysis, the redundant time interval in anormal telephone speech, during which interval the instantaneous speechsignal voltage is substantially zero, amounts to more than one-half ofthe total speech time. With the conventional PPM communications system,pulse emission is maintained at a regular sampling rate even in suchredundant time interval. It may be said Reissuecl Aug. 21, 1973therefore that such conventional PPM communications system has ampleroom for reducing the average transmission power.

A typical approach to reduce the mean transmission power is found in theso-called TASI system, which employs the voice-operated switch. Theswitch is adapted to detect the envelope of the voice input signal andsuppresses the pulse emission at every moment at which the input levelis zero. In order to unfailingly detect the envelope, however, suchswitch must have a comparatively large time constant to follow up thelowest frequency component of the input voice signal. This unavoidablyresults in some defects such as the so-called mutilation of conversationat the rise time of the voice signal. Also, failure of prompt suspensionof pulse emissions is inevitable at every time point immediately afterthe return of the voice signal level to zero.

Accordingly an object of this invention is to provide an improved PPMcommunication system which is capable of eliminating the aforementioneddefects of the conventional PPM systems using voice-operated switches.

Another object of this invention is to provide a PPM system comprisingmeans for suppressing pulse emission at every time point at which thevoice input level becomes substantially zero, such as in the pause inthe normal speech, with a view to considerably saving the averagetransmission power emissions.

Still another object of this invention is to provide a PPM particularlyadapted to the RADA system, which provides a plurality of communicationchannels without resorting to elevation of the average transmissionpower.

FIGS. 1, 2 and 3 are schematic block diagrams of three preferredembodiments of this invention, respectively. FIG. 4 illustratesschematically the timing relationship among typical voltage waveforms atvarious positions in the circuit arrangements shown in FIGS. 1 through3.

Now the principles of this invention will be described with reference tothe appended drawings. Referring to FIGS. 1 and 4, an embodiment of thisinvention shown in FIG. 1 has sampling pulse oscillator unit 11consisting of sampling frequency oscillator 111, Schmitt trigger ofsinusoidal wave into a rectangular wave and first monostablemultivibrator 113 triggered by the output of Schmitt trigger circuit 112for generating a train of regularly circuit 112 for converting thesampling frequency signal spaced narrow-width sampling pulses as shownin FIG. 4(A). A sawtooth-wave generator 12 generates in response to thesampling pulses (A), the sawtooth wave as shown at FIG. 4(B). Thesawtooth wave (B) is applied at input terminal 131A to pulse positionmodulator unit 13 consisting of PWM modulator 131 and a secondmonostable multivibrator 132. A modulating input signal shown in FIG.4(C) is applied to the other terminal 131B, of the modulator, whereincomparison is performed between the sawtooth wave (B) and modulatingsignal (C) for producing a train of PWM pulses during the time intervalin which the modulating signal amplitude is larger than the amplitude ofthe sawtooth wave, as shown in FIG. 4(D). The second monostablemultivibrator 132 generates PPM pulses of extremely short duration, asshown in FIG. 403), at every trailing edge of the PWM pulse (D). The PPMmodulated pulse (E) emerging from the multivibrator 132 is applied to aninput terminal of the inhibit gate 16.

On the other hand, the sampling pulse from the generator 11 is subjectedto delay at delay circuit 14 by one-half of the sampling period T(microseconds). The delayed sampling pulse is applied to a thirdmonostable multivibrator 15, which generates a pulse train having, asshown in FIG. 4(F), the sampling repetition rate and larger width thanthe PPM output pulse (E). The output pulse train (F) of the monostablemultivibrator 15 is supplied to the inhibit input terminal of theinhibit gate 16. Thus, the PPM pulses (E) from the second monostablemultivibrator 132 is inhibited by the output (F) of the monostablemultivibrator 15, every time the pulses (E) coincides with the pulse(F). The output of the inhibit gate 16 is supplied to a fourthmonostable multivibrator 17 provided for shaping purpose. The output ofgate 16 is shown in FIG. 4(G).

To establish the virtual coincidence of the zero-modulation component ofPPM pulse train (E) with the pulse train (F) which is delayed by T/2, aDC bias voltage is always applied to the input terminal 1318 of the PWMmodulator 131. Due to the bias voltage, the modulator 131 generates suchoutput pulses as cause the second monostable multivibrator 132 toproduce corresponding output pulses actually delayed by T/2 at the timepoint where the modulating signal is zero volt.

Assuming that the amplitude of the modulating input signal (C) is notzero, the PPM output (E) will have the constant duration and thevariable spacing as shown at H FIG. 4(E). However, inasmuch as themodulating signal (C) occasionally has zero-volt amplitude even in theduration of a word or a syllable, there will be those zeromodulationcomponents among the pulses of PPM output (E) which should not beinhibited at the gate 16. It is statistically estimated, however, thatsuch zero-modulation components appear quite rarely.

As will be understood from the foregoing, the majority of the PPM outputpulses (E) are delivered to the fourth monostable multivibrator 17Without being inhibited by the inhibit pulses (F) from the thirdmonostable multivibrator 15.

During the time period when the modulating signal (C) becomessubstantially zero as shown by CO in FIG. 4(C), pulses E and E of thepulse train (E) are at the same repetition frequency as the samplingpulses and are delay by 172 with respect to the sampling pulses. Sincepulses E and E coincide with pulses F and F of the pulse train (F), theyare inhibited at the inhibit gate 16 and not delivered to the fourthmonostable multivibrator 17 as shown in FIG. 4(G).

Now an analysis will be made of the reason why the information to betransmitted is substantially unimpaired even when the pulses E and E areremoved.

Even when the modulating signal is present at terminal 1313, some of thePPM pulses (B) may be within the duration t, of the pulses (F) andinhibited at the inhibit gate 16, as mentioned above. The number ofinhibited pulses, however, is extremely small as will be analyzedhereunder. Assuming that the pulse modulation degree is denoted by :t /Zand that the modulating degree signal waveform is sinusoidal, theprobability P that pulses will be inhibited in the total number ofpulses is expressed as Obviously, the number of discarded pulsesincreases as the modulation degree decreases. Also, the probabilitybecomes negligibly small for pulse duration t taken sufliciently smallas compared with t,,,. Furthermore, all pulses are inhibited at theinhibit gate 16 and no output pulses are delivered to the monostablemultivibrator 17 for t gt Accordingly, the pulse duration t, of thepulse train (F) determines the lower limit of the modulating signallevels. Assuming the maximum and the minimum input signal level aredenoted by V and V respectively, the dynamic range D of the present PPMsystem is expressed as max min m c The dynamic range is small for largevalues of t,,. If t is made too small, those pulses E and E of pulsetrain (E) that are to be inhibited will not be suppressedsatisfactorily, because of the presence of noise components contained inthe modulating input signal. Accordingly, a suitable value of t must bepredetermined in relation to t,,,. Communication systems of this kindare normally designed to have dynamic ranges of the order of 10. In thepresent PPM system, assuming that D=l0, the discarded pulse rate will beextremely small, being about 6.36%, for a percent modulation by asinusoidal wave.

Incidentally, the conventional PPM demodulation equipment is applicableas it is to demodulation of the PPM pulses transmitted by theabove-mentioned present PPM system. In other words, any conventional PPMreceiver with the conventional demodulator can be used as thecounterpart of the PPM transmitter described above. On reception by theconventional equipment, it is true that the demodulated signal level islowered, as compared with the reception of the PPM signal from theconventional PPM transmitter the degree of level lowering is howeveronly nominal as will be analyzed hereunder. Let signal levels obtainedby demodulating PPM signal of the present PPM system and a conventionalPPM system be denoted respectively by P, and P Then the relative levelloss factor P which is equal to P.,-P,/P or 1-P,/P may be expressed aswhen t gt (as in the normal case), the above equation can be rewrittenas Since P is only 0.0423% for 11:10, this relative level loss factorcan be substantially neglected. This signifies that pulses of nopractical importance (which do not materially contribute to theinformation transmission) have been discarded in the informationtransmission according to this invention.

In the second embodiment shown in FIG. 2 of this invention, samplingpulse oscillator unit 11, first sawtoothwave generator 12, PPM modulatorunit 13, inhibitor gate 16, and multivibrator 17 are similar to thosementioned in FIG. 1. Instead of the delay means 14 and multivibrator 15,employed in the first embodiment, a second sawtoothwave generator 12'for sawtooth waves of substantially same amplitude and repetition rateas the first sawtoothwave generator 12, and a second PPM modulator unit13' are employed here. The modulator unit 13 consists of PWM modulator131' and a monostable multivibrator 132', which are quite similar to PWMmodulator 131 and monostable multivibrator 132, respectively. Thesawtooth wave (B) supplied from generator 12 through input terminal 131Ais compared at PWM modulator 131' with a DC bias voltage applied to theinput terminal 13113.

The manner in which the PPM pulses (G) are produced in this embodimentis exactly the same as in the first embodiment. In order to product theinhibitor pulse train, use is made of the second sawtooth-wave generator12' and the second PPM modulator unit 13. The DC bias voltage applied tothe input terminal 1313' is made equal to that applied to the inputterminal 1313 to attain the T/ 2 delay. As a result of the amplitudecomparison between the sawtooth wave (B) and the constant D.C. biasvoltage, the inhibitor pulse train shown in FIG. 4(F) is produced by themodulator unit 13'. By use of the inhibition pulse from the modulatorunit 13, the output pulses E and E of the pulse train (E) are inhibitedat the inhibit gate 16.

In the third embodiment shown in FIG. 3, the sawtooth wave (B) from thesawtooth-wave generator 12 is shared by the first and second PPMmodulator units 13 and 13'. All the other elements and their functionsare similar to those employed in the second embodiment. Therefore,further description of the embodiment is omitted.

In the first embodiment, a difference in T/ 2 time delay between thefirst circuit including generator 12 and modulator unit 13, and thesecond circuit including delay 14 and multiw'brator is liable to occur,because the PPM modulator unit 13 is subjected to such ambienttemperature change and source voltage variations which adversely atfectthe amplitude comparison performed at the PWM modulator to the extentthat the relative time positions of zero-modulation components E and Eand the F and F, pulses cannot be maintained at T/Z.

With the second and third embodiment, such problems are practicallysolved, because both the PM modulator units 13 and 13' are subjected tosimilar temperature and voltage variations.

Obviously, the inhibit gate illustrated in any of FIGS. 1, 2 and 3 maybe substituted by any alternative means such that the power supply forthe monostable multivibrator 132 is switched on and off in response tothe output of the pulse train (F).

While the principles of this invention have been described above inconnection with the three preferred embodiments, it will be apparent tothose skilled in the art that various modifications may be made to theinvention.

We claim:

1. A pulse-position-modulation transmission system comprising means forgenerating a sampling pulse train, means for generating in response tosaid sampling pulse train a sawtooth wave, means for amplitude-comparingsaid sawtooth wave and an information signal to produce apulse-width-modulated pulse, means for converting saidpulse-width-modulated pulse into a pulse-position-modulated pulse, meansfor producing in response to said sampling pulse train a cancellingpulse synchronized with a zero-modulation component of saidpulseposition-modulated pulse, said cancelling pulse producing meanscomprising means for delaying said sampling pulse train by apredetermined time period to produce a delayed pulse train, and meansfor providing a DC. bias signal to said amplitude-comparing means toestablish substantial coincidence of said zero-modulation component andsaid delayed pulse train, and means responsive to said cancelling pulsefor substantially suppressing said zero-modulation component.

2. The system as claimed in claim 1 wherein said cancelling pulseproducing means includes second means for amplitude-comparing saidsawtooth wave and a constant voltage to produce a constantlypulse-width-modulated pulse, and means for converting the last-mentionedpulse into a second pulse-position-modulated pulse.

3. A device for pulse position modulating an information signalcomprising means for generating a first train of pulses at a selectedrepetition rate,

means responsive to the train of pulses and the information signal forproviding a pulse position modulated pulse train representative of saidinformation signal, said pulse position modulated pulse train providingpulses at the same repetition rate as said first pulse train, and

means responsive to the first pulse train and said pulse positionmodulated pulse train for deleting selected pulses from the pulseposition modulated pulse train,

said deletion means comprising means responsive to the first pulse trainfor delaying said pulses a preselected time.

said dellaying mean comprising means generating a bias signa meansresponsive [resopnsive] to a sawtooth signal and said bias signal forproducing a delayed pulse when said sawtooth signal and said bias signalreach a predetermined relationship with one another, said bias signalbeing selected on the basis of the pulse position to be deleted from thepulse position modulated pulse train,

6 said delay being selected on the basis of the selected position forwhich pulses are to be deleted from the pulse position modulated pulsetrain.

4. The device as recited in claim 3 wherein the pulse position modulatedpulse train includes a position for pulses indicative of about zerosignal level of the information signal and wherein said delay isselected to delay the first train of pulses the amount necessary tosubstantially synchronize the delayed pulses with said zero signal levelposition.

5. The device as recited in claim 4 wherein the zero signal levelposition is adjusted to occur at about the midposition of the time basefor the selected repetition rate and whereby said delay is adjusted forsaid mid-position.

6. The device as recited in claim 4 wherein the range of pulse positionsin the pulse position modulated pulse train is T seconds, and the pulsewidth of the delayed pulses is T seconds, and wherein the ratio T /T isgreater than or about ten.

7. The device as recited in claim 4 wherein said deleting means furtherincludes an inhibit gate having its inputs coupled to said pulseposition modulated pulse train and the delayed pulses to inhibit theoccurrence of an output pulse when pulses coincide at substantially thesame time at both inputs and provide an output pulse indicative of thepulse position modulated pulse train in the absence of said coincidenceof pulses.

8. The device as recited in claim 4 wherein said pulse positionmodulated pulse train providing means comprises means responsive to thefirst pulse train for generating said sawtooth signal at the samerepetition rate as said first pulse train, means responsive to thesawtooth signal and the information signal for producing a pulse widthmodulation signal representative of the information signal,

means responsive to a transition of the pulse width modulated signal forgenerating a short pulse representative of a pulse in the pulse positionmodulated pulse train.

9. The device as recited in claim 8 wherein the pulse width modulatingmeans comprises an amplitude comparator circuit comparing theinformation signal to the sawtooth signal and wherein the short pulsegenerating means comprises a monostable multivibrator.

10. The device as recited in claim 3 wherein said means responsive tothe bias signal and the sawtooth signal comprises an amplitudecomparator circuit coupled to the bias signal and the sawtooth signal toproduce an output whgn said sawtooth signal exceeds said bias signal, an

a monostable multivibrator responsive to the output of the comparatorcircuit for producing said delayed pulse.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,510,054 6/1950 Alexander et al. 332-9 R X3,153,196 10/1964 McGuire 325-143 3,161,829 12/1964 Schulman 325-l43 X3,274,514 9/1966 Fowlger 332-9 R X 3,351,873 11/1967 Kirnura 332-9 R3,562,671 2/1971 Hon-ma et a1. 325l43 X ALFRED L. BRODY, PrimaryExaminer U.S. Cl. X.R.

